Halogen-free, fireproof, transparent thermoplastic compositions having high thermomechanical strength, in particular for encapsulation in photovoltaic modules

ABSTRACT

A transparent, fireproof thermoplastic composition which is free from halogen compounds and includes a polyamide-block graft copolymer formed by a polyolefin backbone and, on average, at least one polyamide graft. The grafts are attached to the backbone by the radicals of an unsaturated monomer (X) that has a function capable of reacting with a polyamide, and the radicals of the unsaturated monomer (X) are attached to the backbone by grafting or co-polymerisation from the double bond thereof. The composition includes, as a weight percentage of the total composition: 90 to 99 wt % of the polyamide-block graft copolymer, and 1 to 10 wt % of metal salts of phosphoric acid.

FIELD OF THE INVENTION

The invention relates to flame-retardant transparent thermoplasticcompositions having high mechanical and thermomechanical strength, basedon functionalized polyolefins grafted by polyamide units containing atleast metal salts of phosphonic acid, and containing no halogenatedcompounds, nor phosphorus-containing plasticizer or red phosphorus. Thepresent invention also relates to the use of this composition in anencapsulant or encapsulant-backsheet of a photovoltaic module and alsoto the photovoltaic module comprising such an encapsulant.

Global warming, linked to the greenhouse gases released by fossil fuels,has led to the development of alternative energy solutions which do notemit such gases during their operation, such as for example photovoltaicmodules. A photovoltaic module comprises a “photovoltaic cell”, thiscell being capable of converting light energy into electricity.

There are many types of photovoltaic panel structures.

In FIG. 1, a conventional photovoltaic cell has been represented; thisphotovoltaic cell 10 comprises cells 12, one cell containing aphotovoltaic sensor 14, generally based on silicon that is treated inorder to obtain photoelectric properties, in contact with electroncollectors 16 placed above (upper collectors) and below (lowercollectors) the photovoltaic sensor. The upper collectors 16 of one cellare connected to the lower collectors 16 of another cell 12 byconducting bars 18, generally consisting of an alloy of metals. Allthese cells 12 are connected to one another, in series and/or inparallel, in order to form the photovoltaic cell 10. When thephotovoltaic cell 10 is placed under a light source, it delivers acontinuous electric current, which may be recovered at the terminals 19of the cell 10.

With reference to FIG. 2, the photovoltaic module 20 comprises thephotovoltaic cell 10 from FIG. 1 encased in an “encapsulant”, the latterbeing composed of an upper portion 22 and a lower portion 23. An upperprotective layer 24 (known under the term “frontsheet”, usedhereinafter) and a protective layer on the back of the module (knownunder the term “backsheet”, also used hereinafter) 26 are positioned oneither side of the encapsulated cell.

The impact and moisture protection of the photovoltaic cell 10 isprovided by the upper protective layer 24, generally made of glass.

The backsheet 26, for example a multilayer film based on a fluoropolymerand polyethylene terephthalate, contributes to the moisture protectionof the photovoltaic module 20 and to the electrical insulation of thecells 12 to prevent any contact with the outside environment.

The encapsulant 22 and 23 must perfectly adopt the shape of the spaceexisting between the photovoltaic cell 10 and the protective layers 24and 26 in order to avoid the presence of air, which would limit theefficiency of the photovoltaic module. The encapsulant 22 and 23 mustalso prevent contact of the cells 12 with water and oxygen from the air,in order to limit the corrosion thereof. The upper portion 22 of theencapsulant is between the cell 10 and the upper protective layer 24.The lower portion 23 of the encapsulant is between the cell 10 and thebacksheet 26. In one embodiment variant of the encapsulant, there is nolower portion or upper portion so that the cells 12 of the cell are incontact respectively with the backsheet 26 or the frontsheet 24. Thisvariant is illustrated in FIG. 3 where the photovoltaic cell 12 is seenin direct contact with the frontsheet 24 and is described in greaterdetail in particular in application WO 99/04971.

In the presence of solar radiation, a temperature rise is created insidethe solar module and temperatures of 70° C. (or more) may be achieved.The thermomechanical properties, and in particular the creep resistanceof the adhesive, of the binder or of the encapsulant must therefore bemaintained at these temperatures so that the solar module is notdeformed. The creep resistance is more particularly important in thecase of the encapsulant: indeed, in the event of creep, the cell maycome back into contact with air and/or the upper and/or lower protectivelayers, which leads to a reduction in the efficiency of the solarmodule, or even a degradation of the cell and of the solar module.

In order to guarantee a good durability of the solar module, theencapsulant, like the protective layers, must have a good stability inthe presence of moisture and sufficient barrier properties with respectto moisture.

Furthermore, so as not to reduce the efficiency of the solar module, itis necessary for the encapsulant to enable the transmission of the lightwaves of the solar radiation to the cells. Again so as not to reduce theefficiency, it is necessary for these light waves to be barelydiffracted, that is to say that the encapsulant must be, to the nakedeye, transparent or slightly translucent, the transparency beingquantified by its “haze”, which must be low. It is also necessary forthe encapsulant to have good electrical insulation properties, in orderto avoid any short-circuit within the photovoltaic module.

Thus, in the encapsulant or encapsulant-backsheet applications forphotovoltaic modules, the materials or compositions must imperativelyhave a perfect transparency in order to allow the lossless transmission(or transmission with minimal loss) of the light radiation. Furthermore,this thermoplastic composition/material must have a good mechanicalstrength, in particular with respect to the elongation at break andtensile strength properties, and also good thermomechanical properties,in particular with respect to the hot creep test, considering thetemperature rise of the photovoltaic module during prolonged exposure tothe sun. Finally, this thermoplastic composition/material must also havea good level of fire resistance (flame-retardancy) in the case ofbuilding-integrated photovoltaic modules.

Currently, there are no thermoplastic compositions on the market, inparticular for the encapsulants of photovoltaic modules, that havesatisfactory properties with regard to these various requirements thatare firstly the transparency (with a need to remain transparent overtime), then a good mechanical and thermomechanical strength and also agood fire resistance.

Prior Art

Generally, a person skilled in the art knows that the improvement of theflame-retardant properties of a thermoplastic composition generallytakes place at the expense of its mechanical and thermomechanicalproperties. Furthermore, in the field of thermoplasticcompositions/materials, it is known and practiced to add to saidcompositions/materials flame-retardant mineral fillers (aluminumtrihydroxide, magnesium dihydroxide, ammonium polyphosphate, melaminecyanurate, metal salts of phosphinic acid, red phosphorus, brominatedadditives, etc.), but these additions inevitably lead to an opaqueappearance.

To date, only transparent thermoplastic compositions are known providedplasticizers (which are liquid or solid at ambient temperature) areused, for example plasticizers based on phosphate groups orchlorinated/brominated groups. However, these plasticizers, which arelow molecular weight species, are known to give, eventually or underthermal aging conditions, problems of surface migration (“blooming”phenomenon) and of volatility, therefore losses of usage performanceover time, both from a viewpoint of the mechanical/thermomechanicalperformances and from a viewpoint of the transmission of light.

Document U.S. Pat. No. 4,972,011 is thus known, which describes ahalogen-free flame-retardant thermoplastic composition consisting ofpolyamide (or of block copolymers of polyether-block-amide (PEBA) type)comprising phosphonic acid salts. Document US 2006/0138391 is alsoknown, which describes a halogen-free flame-retardant thermoplasticcomposition consisting of an optionally impact-modified polyamide(blends of polyamide and of functional rubbers) comprising phosphonicacid salts and nitrogen-containing additives.

These compositions are not presented as transparent and even if aparticular thermoplastic variety described in these documents is so, itis known that impact-modified PAs give opaque materials insofar as thedispersion of the functional rubber nodules has sizes that are alwaysgreater than 100 nm, inevitably leading to the diffraction of the lightrays and therefore to opaque materials. Furthermore, their mechanicaland thermomechanical qualities are particularly low and certainly arenot sufficient for high tech and long service life applications, such ascertain applications for photovoltaic modules.

BRIEF DESCRIPTION OF THE INVENTION

It has been observed by the applicant, after various experiments andmanipulations, that a halogen-free thermoplastic composition fulfillsall of the properties necessary in particular for high tech applicationsin the photovoltaic field, namely an optimum degree of transparency anda retention of these optical properties over time including whenconfronted with harsh environmental conditions, and excellent mechanicaland thermomechanical properties and also the classification of thiscomposition as a flame-retardant material according to the UL94classification.

Thus, the present invention relates to a flame-retardant and transparentthermoplastic composition that contains no halogenated compound,comprising a polyamide-block graft copolymer consisting of a polyolefinbackbone and, on average, at least one polyamide graft wherein thegrafts are attached to the backbone by the residues of an unsaturatedmonomer (X) having a function capable of reacting with a polyamide, theresidues of the unsaturated monomer (X) are attached to the backbone bygrafting or copolymerization via its double bond, characterized in thatthe composition comprises, as a weight percentage of the totalcomposition:

-   -   between 90% and 99% by weight of the polyamide-block graft        copolymer, and    -   between 1% and 10% by weight of metal salts of phosphonic acid.

Advantageously, the thermoplastic composition of the inventioncomprises:

-   -   between 94% and 97% by weight of the polyamide-block graft        copolymer, and    -   between 3% and 6% by weight of metal salts of phosphonic acid.

According to one distinctive feature of the invention, thepolyamide-block graft copolymer comprises from 10% to 50% by weight ofpolyamide grafts.

According to one distinctive feature of the invention, the molar mass ofthe polyamide grafts is within the range extending from 1000 to 5000g/mol, preferably within the range extending from 2000 to 3000 g·mol⁻¹.Furthermore, the number of monomers (X) attached to the polyolefinbackbone is greater than or equal to 1.3 and/or less than or equal to10.

According to one preferred embodiment of the invention, the unsaturatedmonomer (X) is chosen from a carboxylic acid anhydride and anunsaturated epoxide.

According to one embodiment of the invention, the thermoplasticcomposition comprises only the polyamide-block graft copolymer and ametal salt of phosphonic acid.

According to one variant, the thermoplastic composition of the inventioncomprises at least one coupling agent in order to improve the adhesivestrength of the composition to the substrates of the photovoltaic cellwhen this adhesive strength must be particularly high.

The composition may also comprise at least one of the additionalcomponents chosen from crosslinking agents, UV absorbers, mineralfillers, plasticizers, and coloring or whitening compounds.

The present invention also relates to the use of this thermoplasticcomposition as an encapsulant film for a photovoltaic module.

Finally, the invention also relates to a photovoltaic module having atleast two films, including one film that forms an encapsulant,comprising a photovoltaic cell capable of generating electrical energy,this film being formed by the aforesaid thermoplastic composition.

DESCRIPTION OF THE APPENDED FIGURES

The description which follows is given solely by way of illustration andnonlimitingly with reference to the appended figures, in which:

FIG. 1, already described, represents an example of a photovoltaic cell,the parts (a) and (b) being ¾ views, part (a) showing a cell beforeconnection and part (b) a view after connection of two cells; part (c)is a top view of a complete photovoltaic cell.

FIG. 2, already described, represents a cross section of a photovoltaicmodule, the “conventional” photovoltaic sensor of which is encapsulatedby an upper encapsulant film and a lower encapsulant film.

FIG. 3, already described, represents a cross section of a photovoltaicmodule, the “thin-film” photovoltaic sensor of which, deposited on theupper protective layer, is encapsulated with a lower encapsulant film.

DETAILED DESCRIPTION OF THE INVENTION

Regarding the polyolefin backbone, this is a polymer comprising, asmonomer, an α-olefin.

α-olefins having from 2 to 30 carbon atoms are preferred.

As α-olefin, mention may be made of ethylene, propylene, 1-butene,1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene,3-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene,1-hexadecene, 1-octadecene, 1-eicosene, 1-docosene, 1-tetracosene,1-hexacosene, 1-octacosene, and 1-triacontene. In the context of thepresent invention, the term “α-olefin” also comprises styrene.Propylene, and very especially ethylene, are preferred as α-olefin.

This polyolefin may be a homopolymer when a single α-olefin ispolymerized in the polymer chain. Mention may be made, as examples, ofpolyethylene (PE) or polypropylene (PP).

This polyolefin may also be a copolymer when at least two comonomers arecopolymerized in the polymer chain, one of the two comonomers referredto as the “first comonomer” being an cc-olefin and the other comonomer,referred to as the “second comonomer” is a monomer capable ofpolymerizing with the first monomer.

As the second comonomer, mention may be made of:

-   -   one of the α-olefins already mentioned, the latter being        different from the first α-olefin comonomer,    -   dienes, such as for example 1,4-hexadiene, ethylidene norbornene        and butadiene,    -   unsaturated carboxylic acid esters such as, for example, alkyl        acrylates or alkyl methacrylates grouped together under the term        alkyl(meth)acrylates. The alkyl chains of these (meth)acrylates        may have up to 30 carbon atoms. Mention may be made, as alkyl        chains, of methyl, ethyl, propyl, n-butyl, sec-butyl, isobutyl,        tert-butyl, pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, nonyl,        decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,        hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl,        heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl,        hexacosyl, heptacosyl, octacosyl, nonacosyl. Methyl, ethyl and        butyl (meth)acrylates are preferred as unsaturated carboxylic        acid esters,    -   carboxylic acid vinyl esters. As examples of carboxylic acid        vinyl esters, mention may be made of vinyl acetate, vinyl        versatate, vinyl propionate, vinyl butyrate or vinyl maleate.        Vinyl acetate is preferred as carboxylic acid vinyl ester.

Advantageously, the polyolefin backbone comprises at least 50 mol % ofthe first comonomer; its density may advantageously be between 0.86 and0.96.

The preferred polyolefin backbones consist of an ethylene/alkyl(meth)acrylate copolymer. By using this polyolefin backbone, excellentaging, light and temperature resistance are obtained.

It would not be outside of the scope of the invention if different“second comonomers” were copolymerized in the polyolefin backbone.

According to the present invention, the polyolefin backbone contains atleast one residue of a functional unsaturated monomer (X) that can reactat an acid and/or amine function of the polyamide graft via acondensation reaction. According to the definition of the invention, thefunctional unsaturated monomer (X) is not a “second comonomer”.

As functional unsaturated monomer (X) included in the polyolefinbackbone, mention may be made of:

-   -   unsaturated epoxides. Among these, are for example aliphatic        glycidyl esters and ethers such as allyl glycidyl ether, vinyl        glycidyl ether, glycidyl maleate and glycidyl itaconate,        glycidyl acrylate and glycidyl methacryalte. They are also, for        example, alicyclic glycidyl esters and ethers such as        2-cyclohexene-1-glycidyl ether, glycidyl        cyclohexene-4,5-dicarboxylate, glycidyl        cyclohexene-4-carboxylate, glycidyl        5-norbornene-2-methyl-2-carboxylate and diglycidyl        endo-cis-bicyclo[2.2.1]-5-heptene-2,3-dicarboxylate. As        unsaturated epoxide, glycidyl methacrylate is preferably used.    -   Unsaturated carboxylic acids and their salts, for example        acrylic acid or methacrylic acid and the salts of the same        acids.    -   Carboxylic acid anhydrides. They may be chosen, for example,        from maleic, itaconic, citraconic, allylsuccinic,        cyclohex-4-ene-1,2-dicarboxylic,        4-methylenecyclohex-4-ene-1,2-dicarboxylic,        bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic and        x-methyl-bicyclo[2.2.1]hept-5-ene-2,2-dicarboxylic anhydrides.        As carboxylic acid anhydride, maleic anhydride is preferably        used.

The functional unsaturated monomer (X) is preferably chosen from anunsaturated carboxylic acid anhydride and an unsaturated epoxide. Inparticular, for achieving the condensation of the polyamide graft withthe polyolefin backbone, in the case where the reactive end of thepolyamide graft is a carboxylic acid function, the functionalunsaturated monomer (X) is preferably an unsaturated epoxide. In thecase where the reactive end of the polyamide graft is an amine function,the functional unsaturated monomer (X) is preferably an unsaturatedcarboxylic acid anhydride.

According to one advantageous version of the invention, the preferrednumber of functional unsaturated monomers (X) attached, on average, tothe polyolefin backbone is greater than or equal to 1.3 and/orpreferably less than or equal to 10.

When (X) is maleic anhydride and the number-average molar mass of thepolyolefin is 15 000 g/mol, it was found that this corresponded to ananhydride proportion of at least 0.8%, and at most 6.5%, by weight ofthe whole of the polyolefin backbone. These values associated with themass of the polyamide grafts determine the proportion of polyamide andof backbone in the polyamide graft polymer.

The polyolefin backbone containing the residue of the functionalunsaturated monomer (X) is obtained by polymerization of the monomers(first comonomer, optional second comonomer, and optionally functionalunsaturated monomer (X)). This polymerization can be carried out by ahigh-pressure radical process or a process in solution, in an autoclaveor tubular reactor, these processes and reactors being well known to aperson skilled in the art. When the functional unsaturated monomer (X)is not copolymerized in the polyolefin backbone, it is grafted to thepolyolefin backbone. The grafting is also an operation that is known perse. The composition would be in accordance with the invention if severaldifferent functional monomers (X) were copolymerized with and/or graftedto the polyolefin backbone.

Depending on the types and ratio of monomers, the polyolefin backbonemay be semicrystalline or amorphous. In the case of amorphouspolyolefins, only the glass transition temperature is observed, whereasin the case of semicrystalline polyolefins a glass transitiontemperature and a melting temperature (which will inevitably be higher)are observed. A person skilled in the art will only have to select theratios of monomer and the molecular masses of the polyolefin backbone inorder to be able to easily obtain the desired values of the glasstransition temperature, optionally of the melting temperature, and alsoof the viscosity of the polyolefin backbone.

Preferably, the polyolefin has a melt flow index (MFI) between 3 and 400g/10 min (190° C./2.16 kg, ASTM D 1238).

The polyamide grafts may be either homopolyamides or copolyamides.

The expression “polyamide grafts” especially targets the aliphatichomopolyamides which result from the polycondensation:

-   -   of a lactam;    -   or of an aliphatic α,ω-aminocarboxylic acid;    -   or of an aliphatic diamine and an aliphatic diacid.

As examples of a lactam, mention may be made of caprolactam,oenantholactam and lauryllactam.

As examples of an aliphatic α,ω-aminocarboxylic acid, mention may bemade of aminocaproic acid, 7-aminoheptanoic acid, 11-aminoundecanoicacid and 12-aminododecanoic acid. As examples of an aliphatic diamine,mention may be made of hexamethylenediamine, dodecamethylenediamine andtrimethylhexamethylene-diamine.

As examples of an aliphatic diacid, mention may be made of adipic,azelaic, suberic, sebacic and dodecanedicarboxylic acids.

Among the aliphatic homopolyamides, mention may be made, by way ofexample and nonlimitingly, of the following polyamides: polycaprolactam(PA-6); polyundecanamide (PA-11, sold by Arkema under the brandRilsan®); polylauryllactam (PA-12, also sold by Arkema under the brandRilsan®); polybutylene adipamide (PA-4,6); polyhexamethylene adipamide(PA-6,6); polyhexamethylene azelamide (PA-6,9); polyhexamethylenesebacamide (PA-6,10); polyhexamethylene dodecanamide (PA-6,12);polydecamethylene dodecanamide (PA-10,12); polydecamethylene sebacamide(PA-10,10) and polydodecamethylene dodecanamide (PA-12,12).

The expression “semicrystalline polyamides” also targets cycloaliphatichomopolyamides.

Mention may especially be made of the cycloaliphatic homopolyamideswhich result from the condensation of a cycloaliphatic diamine and analiphatic diacid.

As an example of a cycloaliphatic diamine, mention may be made of4,4′-methylenebis(cyclohexylamine), also known aspara-bis(aminocyclohexyl)methane or PACM,2,2′-dimethyl-4,4′-methylenebis(cyclohexylamine), also known asbis(3-methyl-4-aminocyclohexyl)-methane or BMACM.

Thus, among the cycloaliphatic homopolyamides, mention may be made ofthe polyamides PACM,12 that results from the condensation of PACM withthe C12 diacid, BMACM,10 and BMACM,12 that result from the condensationof BMACM with, respectively, C10 and C12 aliphatic diacids. Theexpression “polyamide grafts” also targets the semiaromatichomopolyamides that result from the condensation:

-   -   of an aliphatic diamine and an aromatic diacid, such as        terephthalic acid (T) and isophthalic acid (I). The polyamides        obtained are then commonly known as “polyphthalamides” or PPAs;        and    -   of an aromatic diamine, such as xylylenediamine, and more        particularly meta-xylylenediamine (MXD) and an aliphatic diacid.

Thus, nonlimitingly, mention may be made of the polyamides 6,T, 6,l,MXD,6 or else MXD,10.

The polyamide grafts used in the composition according to the inventionare preferably copolyamides. These result from the polycondensation ofat least two of the groups of monomers mentioned above in order toobtain homopolyamides. The term “monomer” in the present description ofthe copolyamides should be taken in the sense of a “repeat unit”. Thisis because the case where a repeat unit of the PA is formed from thecombination of a diacid with a diamine is particular. It is consideredthat it is the combination of a diamine and a diacid, that is to say thediamine-diacid pair (in an equimolar amount), which corresponds to themonomer. This is explained by the fact that individually, the diacid orthe diamine is only one structural unit, which is not enough on its ownto polymerize in order to give a polyamide.

Thus, the copolyamides cover especially the condensation products of:

-   -   at least two lactams;    -   at least two aliphatic α,ω-aminocarboxylic acids;    -   at least one lactam and at least one aliphatic        α,ω-aminocarboxylic acid;    -   at least two diamines and at least two diacids;    -   at least one lactam with at least one diamine and at least one        diacid;    -   at least one aliphatic α,ω-aminocarboxylic acid with at least        one diamine and at least one diacid,

the diamine(s) and the diacid(s) possibly being, independently of oneanother, aliphatic, cycloaliphatic or aromatic.

Depending on the types and ratio of monomers, the copolyamides may besemicrystalline or amorphous. In the case of amorphous copolyamides,only the glass transition temperature is observed, whereas in the caseof semicrystalline copolyamides a glass transition temperature and amelting temperature (which will inevitably be higher) are observed.

Among the amorphous copolyamides that can be used within the context ofthe invention, mention may be made, for example, of the copolyamidescontaining semiaromatic monomers.

Among the copolyamides, it is also possible to use semicrystallinecopolyamides and particularly those of the PA-6/11, PA-6/12 andPA-6/11/12 type.

The degree of polymerization may vary to a large extent; depending onits value it is a polyamide or a polyamide oligomer.

Advantageously, the polyamide grafts are monofunctional.

So that the polyamide graft has a monoamine end group, it is sufficientto use a chain limiter of formula:

in which:

-   -   R₁ is hydrogen or a linear or branched alkyl group containing up        to 20 carbon atoms; and    -   R₂ is a group having up to 20 carbon atoms that is a linear or        branched alkyl or alkenyl group, a saturated or unsaturated        cycloaliphatic 2 0 radical, an aromatic radical or a combination        of the preceding. The limiter may be, for example, butylamine,        hexylamine, heptylamine, octylamine, decylamine, laurylamine or        oleylamine.

So that the polyamide graft has a carboxylic monoacid end group, it issufficient to use a chain limiter of formula R′1-COOH, R′1-CO-O-CO-R′2or a carboxylic diacid.

R′1 and R′2 are linear or branched alkyl groups containing up to 20carbon atoms.

Advantageously, the polyamide graft has one end group having an aminefunctionality. The preferred monofunctional polymerization limiters arelaurylamine and oleylamine.

Advantageously, the polyamide grafts have a molar mass between 1000 and5000 g/mol and preferably between 2000 and 3000 g/mol.

The polycondensation defined above is carried out according to commonlyknown processes, for example at a temperature generally between 200° C.and 350° C., under vacuum or in an inert atmosphere, with stirring ofthe reaction mixture. The average chain length of the graft isdetermined by the initial molar ratio between the polycondensablemonomer or the lactam and the monofunctional polymerization limiter. Forthe calculation of the average chain length, one chain limiter moleculeis usually counted per one graft chain.

A person skilled in the art will only have to select the types and ratioof monomers and also choose the molar masses of the polyamide grafts inorder to be able to easily obtain the desired values of the glasstransition temperature, optionally of the melting temperature and alsoof the viscosity of the polyamide graft.

The condensation reaction of the polyamide graft on the polyolefinbackbone containing the residue of (X) is carried out by reaction of oneamine or acid function of the polyamide graft with the residue of (X).Advantageously, monoamine polyamide grafts are used and amide or imidebonds are created by reacting the amine function with the function ofthe residue of (X).

This condensation is preferably carried out in the melt state. Tomanufacture the composition according to the invention, it is possibleto use conventional kneading and/or extrusion techniques. The componentsof the composition are thus blended to form a compound which mayoptionally be granulated on exiting the die.

To obtain the graft copolymers of the invention, it is thus possible toblend the polyamide graft and the backbone in an extruder, for example aself-cleaning meshing co-rotating twin-screw extruder, at a temperaturegenerally between 150° C. and 300° C. The average residence time of themolten material in the extruder may be between 5 seconds and 5 minutes,and preferably between 20 seconds and 1 minute. The efficiency of thiscondensation reaction is evaluated by selective extraction of freepolyamide grafts, that is to say those that have not reacted to form thepolyamide graft polymer.

The preparation of polyamide grafts having an amine end group and alsotheir addition to a polyolefin backbone containing the residue of (X) isdescribed in patents U.S. Pat. No. 3,976,720, U.S. Pat. No. 3,963,799,U.S. Pat. No. 5,342,886 and FR 2291225.

The polyamide graft polymer of the present invention advantageously hasa nanostructured organization. To obtain this type of organization, usewill preferably be made, for example, of grafts having a number-averagemolar mass M_(n) between 1000 and 5000 g/mol and more preferably between2000 and 3000 g/mol. Use will also preferably be made of between 10% and50% by weight of polyamide grafts and a number of monomers (X) between1.3 to 10.

Advantageously, the flow temperature of the polyamide graft polymer isless than or equal to 160° C., which permits a processing temperaturethat is particularly well suited to the current techniques formanufacturing solar panels.

Regarding the metal salts of phosphonic acid, these may be any saltsfrom among the metal salts of alkyl alkylphosphonic acid of generalformula (I):

Where X is a metal atom and R¹ and R² are the same or different linearor branched alkyl groups possessing from 1 to 12 carbon atoms,preferably from 1 to 4 carbon atoms, such as the examples below givennonlimitingly: methyl, ethyl, propyl, isopropyl, butyl, isobutyl,sec-butyl; n being equal to the valence number of the metal X, whichlies between 1 and 4, preferably between 2 and 3. Hereinafter, relativeto the test examples, use is made of methyl methylphosphonic acid, butit is clearly understood that a similar result would be obtained withany metal salts of alkyl alkylphosphonic acid, according to thespecifications set out above.

The metals X of the above formula capable of being present in the metalsalts of alkyl alkylphosphonic acid comprise alkaline metals ortransition metals such as the group below given nonlimitingly: Ca(calcium), Mg (magnesium), Zn (zinc), Al (aluminum), Fe (iron), Ni(nickel), Cr (chromium) and Ti (titanium).

Preferably, the metal salt of phosphonic acid is the aluminum salt ofmethyl methylphosphonic acid (AMMP), where X is the aluminum atom, R¹and R² are methyl groups and n is equal to 3. AMMP contains a highdegree of active phosphorus (26% by weight). AMMP can be synthesizedeither by reacting methyl methylphosphonate with an aqueous solution ofsodium hydroxide then by precipitating it with aluminum chloride, or bythe direct reaction of aluminum hydroxide with methyl methylphosphonateat 180° C. under vigorous stirring.

Preferably, the metal salt of alkyl alkylphosphonic acid is in the formof a powder with particles sizes of less than 25 μm (micrometer), morepreferably of less than 10 μm and more preferably still less than 5 μm.The preferred form of the metal salt of alkyl alkylphosphonic acidcomprises a distribution of particles between 0.1 and 3 μm, without thepresence of particles having a size of less than 0.1 μm.

Fillers, in particular mineral fillers, may be added in order to improvethe thermomechanical strength of the composition. Given nonlimitingly asexamples are silica, alumina or calcium carbonates or carbon nanotubes.Advantageously, use will be made of nanoscale fillers (organophilicclays or carbon nanotubes) which are dispersed on the nanometer scale;this makes it possible to retain, in the best cases, the transparency ofthe materials.

It is also possible to add coloring or whitening compounds.

Regarding the aspects of the invention relating to the use of thethermoplastic composition in a photovoltaic module, a person skilled inthe art may refer, for example, to the Handbook of Photovoltaic Scienceand Engineering, Wiley, 2003. Indeed, the composition of the inventionmay be used as an encapsulant or encapsulant-backsheet in a photovoltaicmodule, the structure of which is described in relation to the appendedfigures.

Among the list of additives below, a person skilled in the art willeasily know how to select their amounts in order to obtain the desiredproperties of the composition, in particular in its application inphotovoltaic modules.

Coupling agents, although not necessary, may advantageously be added inorder to improve the adhesive strength of the composition on thesubstrates of the photovoltaic cell when this adhesive strength must beparticularly high. The coupling agent is a non-polymeric ingredient; itmay be organic, crystalline, mineral and more preferably semi-mineralsemi-organic. Among the latter, mention may be made of organic titanatesor silanes, such as for example monoalkyl titanates, trichlorosilanesand trialkoxysilanes.

Although crosslinking is not obligatory, it is possible for furtherimproving the thermomechanical properties of the encapsulant, inparticular when the temperature becomes very high. It is not thereforeoutside the scope of the invention if crosslinking agents are added.Mention may be made, by way of example, of organic peroxides. This crosslinking may also be carried out by known irradiation techniques.

Preferably, the composition comprises no more than 10% tackifying resinand preferably does not contain any. Indeed, when these resins are addedto the polyamide graft polymer, the transparency of the composition andthe creep resistance decrease. These tackifying resins are, for example,rosins and derivatives thereof, polyterpenes and derivatives thereof.Surprisingly, no tackifying resin is necessary for giving thecomposition properties of adhesion to the various supports of solarmodules.

In this particular application of the composition in photovoltaicmodules, since the UV radiation is capable of resulting in a slightyellowing of the composition used as an encapsulant for said modules, UVstabilizers may be added in order to ensure the permanence/longevity ofthe transparency of the encapsulant during its service life. Thesecompounds may be, for example, based on benzophenone or benzotriazole.They can be added in amounts of less than 10%, and preferably of from0.1% to 5%, by weight of the total weight of the composition.

Obtaining the Formulations Tested:

The formulations described below are prepared by compounding using, forexample, a Coperion® ZSK30 self-cleaning meshing co-rotating twin-screwextruder having a diameter of 30 millimeters (mm), a length 44 times itsdiameter (i.e. 132 centimeters) with a flat profile at 200° C., with athroughput of 20 kg/h (kilograms per hour) and a rotational speed of 300rpm (revolutions per minute), the polymers and the additives in powderform being introduced as main feed.

Generally, the term “compounding” is understood to mean a technique forobtaining polymers or blends of polymers that is well known to a personskilled in the art and which consists of a shaping of the formulate(present for example in the form of rods on leaving the kneader) byextrusion through a die having circular holes, then cutting of thecooled rods and drying in order to manufacture granules that are a fewmillimeters in diameter and in length.

Materials Used for Forming the Formulations Tested:

Apolhya®: graft copolymer with a backbone composed of a terpolymer ofethylene, ethyl acrylate (EA) and maleic anhydride (MAh) possessing 17%of EA, 3% of MAh and an MFI (190° C., 2.16 kg) of 70 and possessing 12/6(60/40) monoamine copolyamide grafts having an Mn of 2500 g/mol.

AF069: aluminum salt of methyl methylphosphonic acid, sold by ICL.

Exolit® AP766: ammonium polyphosphate produced by Clariant and having aphosphorus content of 21% and a nitrogen content of 12%.

Siliporite® NK10AP: molecular sieve of zeolite 4A type produced by CECA.

Lotader®: terpolymer of ethylene, ethyl acrylate (EA) and maleicanhydride (MAh) having 17% of EA, 3% of MAh and an MFI (190° C., 2.16kg) of 70.

Domamid® 24: PA-6 sold by DOMO, having a relative viscosity of 2.4measured at 1% in 96% sulfuric acid according to the standard ISO 307.

Melapur® C25: melamine cyanurate sold by CIBA-BASF.

The present invention is illustrated in greater detail by the followingnonlimiting examples.

EXAMPLE 1

the composition entitled “DM1” is a formulation that comes under thescope of the present invention. It comprises, as a weight percentage ofthe total composition, 90.8% of Apolhya® and 9.2% of AF069.

EXAMPLE 2

the composition entitled “DM2” is a formulation that comes under thescope of the present invention. It comprises, as a weight percentage ofthe total composition, 98.5% of Apolhya® and 1.5% of AF069.

EXAMPLE 3

the composition entitled “DM3” is a formulation that comes under thescope of the present invention. It comprises, as a weight percentage ofthe total composition, 96% of Apolhya® and 4% of AF069.

EXAMPLE 4

the composition entitled “DM4” is a formulation that comes under thescope of the present invention. It comprises, as a weight percentage ofthe total composition, 94.3% of Apolhya® and 5.7% of AF069.

EXAMPLE 5

the composition entitled “DM5” is a formulation that comes under thescope of the present invention. It comprises, as a weight percentage ofthe total composition, 96.6% of Apolhya® and 3.4% of AF069.

In order to compare the examples of the invention with respect tothermoplastic compositions from the prior art, the following examples ofcompositions were proposed for the tests.

EXAMPLE 6

the composition entitled “DM6” is a formulation comprising, as a weightpercentage of the thermoplastic composition, 100% of Apolhya®.

EXAMPLE 7

the composition entitled “DM7” is a formulation comprising, as a weightpercentage of the thermoplastic composition, 78% of Apolhya®, 20% ofExolit® AP766 and 2% of Siliporite® NK10AP.

EXAMPLE 8

the composition entitled “DM8” is a formulation comprising, as a weightpercentage of the thermoplastic composition, 86% of Apolhya® and 14% ofAF069.

EXAMPLE 9

the composition entitled “DM9” is a formulation comprising, as a weightpercentage of the thermoplastic composition, 99.6% of Apolhya® and 0.4%of AF069.

EXAMPLE 10

the composition entitled “DM10” is a formulation comprising, as a weightpercentage of the thermoplastic composition, 6% of AF069, 10% of MelapurMC25, 59% of Lotader and 25% of Domamid 24.

Tests Carried Out on the Test Specimens:

The granules resulting from the synthesis processes are shaped using alaboratory twin-screw extruder of ThermoHaake Rheocord System 40 typeequipped with a sheet die; the extruder being heated at 210° C. in orderto give strips, from which the test specimens necessary in order tocharacterize the materials will be cut, with a punch.

The tests that make it possible to characterize the mechanicalproperties of a standard test specimen of a thermoplastic compositionconsist, as representatively as possible, in carrying out a tensile testaccording to the standard ISO R527:93-1BA in order to measure theelongation at break and tensile strength of these thermoplasticmaterials.

The value of the elongation at break defines the capacity of a materialto elongate before breaking when it is stressed under tension. Amaterial is considered to have good mechanical properties from the pointof view of its ductility/brittleness when its elongation at break valueis greater than 100% and when its tensile strength value is greater than7 MPa.

The creep test in an oven (at high temperature and under a pressuregreater than atmospheric pressure) for a certain duration comes next. Inthis particular case, this creep test of the test specimens of IFC(French Institute of Rubber) type cut from the films is carried out at120° C. under a load of 0.5 bar for 15 minutes and consists in measuringthe elongation undergone by the test specimens. If the test specimenyields under the load, the time for arriving at this failure is noted.This test is known to a person skilled in the art under the name “hotset test”.

The resistance to flame propagation is then measured according to theUL94 test according to the standard ISO 1210 on test specimens having athickness of 1.6 mm (millimeter). This test consists of a doubleapplication of a standardized flame in order to determine theextinguishing times on the one hand and to verify on the other hand thegeneration or otherwise of flaming drips (FD) or of non-flaming drips(NFD), this test being repeated 5 times in order to consolidate theexperiment. It will be noted that, in order to satisfy the VOclassification according to the UL 94 flame propagation test, it ispossible for non-flaming drips (NFD) to appear, but in no case forflaming drips (FD) to appear.

In the targeted application, the UL94-V2 classification is expected.

Essential tests are then carried out on the characteristics oftransparency of the thermoplastic composition but also of yellownessindex and of haze. All the test specimens were subjected to tests formeasuring the light transmitted in the spectral region coveringwavelengths between 360 nm and 830 nm and the percentage of this lighttransmitted was measured for each of the test specimens according to thestandard ASTM D1003.

In the targeted application, a percentage of transmission of visiblelight, typically in a wavelength range from 400 to 800 nm, that is ashigh as possible and is at least 85% is expected/desired.

This test is supplemented by the measurement of the “yellowness index”which measures, after a certain time period and in a particularenvironment, the yellowness index of the test specimen.

In the targeted application, a yellowness index that is as low aspossible, less than 5 and preferably less than 2, is expected/desired.

The haze tests consists in measuring the light transmitted through atest specimen. This transmitted light is measured using a nephelometeror a spectrophotometer. The degree of the haze is measured according tothe standard ASTM D1003 with an illuminant C under 2°.

In the targeted application, a haze value that is as low as possible,less than 10 and preferably less than 5 is expected/desired.

All these tests are carried out conventionally by taking standard testspecimens of identical shape for each composition tested and by makingthem undergo each test on a test bench, according to the definitions(shape, dimensions, test speeds, calibration of the machine, accuracy ofthe apparatus, etc.) given by the international standards and that arewell known to a person skilled in the art.

The composition must satisfy all of the aforementioned tests in anoptimal manner in order to be considered to be satisfactory from thepoint of view of its mechanical properties (elongation at break andtensile strength), its thermomechanical properties or in other words itscreep resistance at high temperature (hot creep), its flame-retardantproperties (UL94 classification) and finally with regard to itstransparency. It is clearly understood that the difficulty consists infinding a composition that exhibits good performances for all of theproperties tested and that a single one of these properties at a levelbelow the requirement of the application is enough to disqualify thiscomposition.

As can be observed, the applicant observed, after its experimentations,that, surprisingly, the composition according to the invention perfectlysatisfies all of the tests demonstrating that its mechanical,thermomechanical and flame-retardant properties are excellent, or inother words of a very high level.

The compositions according to the invention therefore fulfill thecriteria for being able to be very advantageously used asflame-retardant binder or encapsulant in solar modules.

Results of the Tests Carried Out on the Test Specimens of VariousFormulations:

DM1 DM2 DM3 DM4 DM5 DM6 DM7 DM8 DM9 DM10 APOLHYA 90.8 98.5 96 94.3 96.6100 78 86 99.6 AF069 9.2 1.5 4 5.7 3.4 14 0.4 6 Exolit AP776 20Siliporite NK10AP 2 Melapur MC25 10 Lotader 59 Domamid 24 25 TESTSElongation at break (%) 290 420 439 380 430 436 252 300 410 42 Tensilestrength (MPa) 9.8 12 11.9 11.6 12.1 12.3 6 9 11.9 4 Hot creep (% ortime) 11% 18% 17% 15% 19% 20% 10% 14% 20% 5 min UL94 classification V2V2/NC V2 V2 V2 NC* V0 V2 NC* V0 Light transmission (%) 85.2 86.4 86.185.8 86.2 86.6 20 74 86.4 15 Yellowness index (%) 3.9 3.3 3.4 3.4 3.23.2 6 8 3 9 Haze (%) 8 3 6 7 6 2 100 18 2 100 **: NC = not classified

By carrying out these first experimentations, the applicant noted thatthe addition of AF069 was particularly prejudicial to the transparencyof an initially transparent or semi-transparent polymer. Presented belowis a table of results with the polymer PC Makrolon, Rilsan G830,modified Altuglas V825T or Apolhya Solar.

Transmittance through Apolhya LC3 Makrolon 2207 Rilsan G830 AltuglasV825T films (illuminant D65, +4% +4% +4% +4% angle 2°) Pure AF069 PureAF069 Pure AF069 Pure AF069 Thickness (μm) 600  550 60   60 50   90 160 130 Transmittance at 89.9 87.5 (−3%) 90.1 82.7 (−8%) 91.6 85.3 (−7%)89.9 86.6 (−4%) 560 nm Yellowness index (YI) 1.7   3 (×1.7) 0.5  1.6(×3) 0.4  2.4 (×6.2) 0.9  2.4 (×2.6) (according to ASTM E313 D65) Haze(according to 8   15 (×1.9) 1   40 (×4) 1   20 (×20) 7   19 (×2.9) ASTMD1003-97 C)

The products used for carrying out the studies and the analyses beloware the following:

Apolhya ® LC3: thermoplastic based on a nanostructured copolymer havinga melting temperature of 130° C. and a MFI value of 15 to 20 g/10 minaccording to ASTM D1238 at 190° C. under 2.16 kg, produced and sold byArkema;

Makrolon® 2207: low-viscosity polycarbonate having an MFI value of 38according to ASTM D1238 at 300° C. under 1.2 kg, produced and sold byBayer;

Rilsan® G830: bio-based transparent polyamide having a glass transitiontemperature of 135° C., a content of renewable carbon of from 53% to55%, produced and sold by Arkema;

Altuglas® V825T: PMMA having an MFI value of 3.8 according to ASTM D1238under 3.8 kg at 230° C. and a Vicat softening temperature of 101° C.according to ASTM D1525, produced and sold by Altuglass.

These results show that the addition of a significant amount of AF069(4%) considerably reduces the optical properties of the polymermaterials. In particular, this is true for Makrolon 2207 and for RilsanG830 and to a lesser extent, but also significantly, for Altuglas V825Tand Apolhya LC3.

It is therefore obvious that a person skilled in the art, in regard tothese first results, cannot envisage that the addition of AF069 could beconceivable for improving the properties of transparent polymermaterials, without greatly impairing their transparency properties(transmittance, yellowness index and haze) and it is only after a largenumber of experiments that the applicant has arrived at an optimumresult, in the particular range of AF069 added to the base thermoplasticcomposition.

1. A flame-retardant and transparent thermoplastic composition thatcontains no halogenated compound, the composition comprising apolyamide-block graft copolymer consisting of a polyolefin backbone and,on average, at least one polyamide graft wherein the grafts are attachedto the backbone by the residues of a functional unsaturated monomer (X)having a function capable of reacting with a polyamide, the residues ofthe unsaturated monomer (X) are attached to the backbone by grafting orcopolymerization via its double bond, the composition comprises, as aweight percentage of the total composition: between 90% and 99% byweight of the polyamide-block graft copolymer, and between 1% and 10% byweight of a metal salt of phosphonic acid.
 2. The composition as claimedin claim 1, wherein the composition comprises: between 94% and 97% byweight of the polyamide-block graft copolymer, and between 3% and 6% byweight of a metal salt of alkyl alkylphosphonic acid.
 3. The compositionas claimed in claim 1, wherein the polyamide-block graft copolymercomprises from 10% to 50% by weight of polyamide grafts.
 4. Thecomposition as claimed in claim 1, wherein the molar mass of thepolyamide grafts is within the range extending from 1000 to 5000 g/mol.5. The composition as claimed in claim 1, wherein the number of monomers(X) attached to the polyolefin backbone is greater than or equal to 1.3and/or less than or equal to
 10. 6. The composition as claimed in claim1, wherein the unsaturated monomer (X) is chosen from a carboxylic acidanhydride and an unsaturated epoxide.
 7. The composition as claimed inclaim 1, wherein the composition comprises only the polyamide-blockgraft copolymer and a metal salt of alkyl alkylphosphonic acid.
 8. Thecomposition as claimed in claim 1, wherein said composition comprises,in addition, at least one coupling agent.
 9. An encapsulant film for aphotovoltaic module, the film comprising the composition as claimed inclaim
 1. 10. A photovoltaic module having at least two films, includingat least one film that forms an encapsulant, comprising a photovoltaiccell capable of generating electrical energy, the encapsulant film beingformed by the thermoplastic composition as claimed in claim
 1. 11. Thecomposition as claimed in claim 1, wherein the molar mass of thepolyamide grafts is within the range extending from 2000 to 3000g·mol⁻¹.